1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device that prevents picture quality from deteriorated by distortion of a common voltage.
2. Discussion of the Related Art
Recently, active matrix LCD devices have been widely incorporated into flat panel TVs, portable computers, monitors, and other electronic applications as their performance have been improved. Among the active matrix LCD devices, a twisted nematic (TN) mode LCD device is mainly used. The TN mode LCD device drives a liquid crystal director twisted at an angle of 90° by applying a voltage to electrodes arranged on two substrates. The TN mode LCD device provides excellent contrast and color reproduction but suffers from a narrow viewing angle.
To solve the narrow viewing angle problem of the TN mode LCD device, an in-plane switching (IPS) mode LCD device has been developed. In the IPS mode LCD device, two electrodes are formed on one substrate and a liquid crystal director is controlled by the IPS mode generated between the two electrodes. The IPS mode LCD device provides a wide viewing angle but suffers from low aperture ratio and transmittance of light.
To improve the low aperture ratio and transmittance of the IPS mode LCD device, a fringe field switching (FFS) mode LCD device has been developed. In the FFS LCD device, a counter electrode and a pixel electrode are formed of transparent conductors, and the distance between the counter electrode and the pixel electrode is maintained at a narrow range to drive liquid crystal molecules using a fringe field formed between the counter electrode and the pixel electrode.
An FFS mode LCD device of the related art includes a color filter array substrate provided with color filter layers, and a thin film transistor array substrate provided with thin film transistors (TFTs), counter electrodes and pixel electrodes. The color filter array substrate and the TFT array substrate are bonded to each other with a liquid crystal layer therebetween. The TFT array substrate, as shown in FIG. 1, includes gate lines 12, data lines 15, TFTs at each intersection of the gate line 12 and data line 15, common lines 25, plate type counter electrodes 24, and pixel electrodes 17. The gate lines 12 and the data lines 15 are formed of opaque metal. The gate lines 12 perpendicularly cross the data lines 15 to define sub-pixels. Each of the TFTs switches on/off a voltage at each crossing point between the respective gate and data lines 12 and 15. The counter and pixel electrodes 24 and 17 are formed of transparent metal, insulated from each other by an insulating layer and overlapped with each other in the pixels. The counter electrode 24 contacts the common lines 25 to receive common signals (Vcom) from the common lines 25.
More specifically, each counter electrode 24 is formed of plate type transparent metal. Each pixel electrode 17 is provided with a plurality of slits 60 symmetrical to one another around a center portion of a pixel region. A fringe field occurs between the counter electrode 24 and the pixel electrode 17 when voltage is applied to the electrodes. In particular, the signal Vcom is transmitted to the counter electrode 24 and a pixel voltage passing through the TFT is transmitted to the pixel electrode 17.
Each of the slits 60 typically has a width of 2 μm to 6 μm. Liquid crystals are driven by the fringe field formed between the pixel electrode 17 and the counter electrode 24. That is, the liquid crystals initially aligned by rubbing in a direction when there is no voltage are rotated by the fringe field to transmit light therethrough.
The color filter array substrate includes red, green, and blue (R/G/B) color filter layers (not shown) arranged at constant intervals to display colors. A black matrix layer serves to divide R/G/B cells from one another and shield aberrant light. The respective color filter layers are formed to correspond to the sub-pixels so that each of the sub-pixels has one color. Conventionally, pixels having R/G/B colors are arranged and independently driven. A color of one pixel is displayed by combination of the R/G/B color of the sub-pixels.
The R/G/B color filter layers are arranged in various patterns, such as a stripe arrangement, a mosaic arrangement, a delta arrangement, or a quad arrangement. The R/G/B color filter layers are arranged depending on the size of an LCD panel, shape of the color filter layer, and color arrangement. The stripe arrangement, as shown in FIG. 2 and FIG. 3, has the R/G/B color filter layers arranged sequentially in a horizontal direction and the same color arranged in a vertical direction.
The related art LCD device as described above is turned on/off per each RIG/B pixels to display black (B) or white (W) to check the picture quality, such as residual images, flicker, and greenish tint. As shown in FIG. 2, the related art LCD device may be driven in a counter pattern—i.e., an Nth turned-on pixel and an N+1th turned-on pixel are horizontally shifted one space per line such that the pixels are turned on in an oblique direction. As shown in FIG. 3, the LCD device may also be driven in a vertical pattern—i.e., an Nth turned-on pixel and an N+1th turned-on pixel are arranged at the same position with each other per line such that the pixels are turned on in a vertical direction.
In the case where the LCD device is driven in the counter pattern, as shown in FIG. 4A, voltages of positive polarity (+) and negative polarity (−) are applied in a horizontal direction according to a one-dot inversion mode and the voltages are applied in a vertical direction according to a two-dot inversion mode. Specifically, a data voltage Vdata is applied to the Nth line, as shown in FIG. 4B, in such a manner that voltages of positive polarity (+) and negative polarity (−) applied using an alternating current (AC) voltage and levels of the data voltage are varied to display black and white. A common voltage Vcom1 applied to the Nth line is a direct current (DC) voltage and the liquid crystal layer is driven by the potential difference between the data voltage Vdata and the common voltage Vcom1.
However, the related art LCD device has several problems. As shown in FIG. 4B, the data voltage Vdata applied to the Nth line is an AC voltage and the common voltage Vcom applied thereto is a DC voltage. Fluctuation of the common voltage Vcom1 is amplified at a portion where the data line 12 overlaps the common line 25 due to common line capacitance Cdc formed between the data line 12 and the common line 25. For this reason, coupling occurs in which the common voltage Vcom1 becomes distorted common voltage Vcom2.
In one pixel having R/G/B sub-pixels to display white, the distorted common voltage Vcom2 of the R sub-pixel and the distorted common voltage Vcom2 of the G sub-pixel are offset by each other. However, the distorted common voltage Vcom2 of the B sub-pixel remains without offset. As a result, the total common voltage is increased by the remaining common voltage than the applied common DC voltage.
As shown in FIG. 5, if the common voltage Vcom2 flowing in the common line 25 due to parasitic coupling increases the applied common DC voltage Vcom1, the voltage difference V2 between the voltages Vdata and Vcom2 applied to the green pixel region is greater than the voltage difference V1 between the voltages Vdata and Vcom2 applied to the red and blue pixel regions. The result is a greenish tint in the displayed image as the color green appears brighter than the other colors. Green appears brighter because rotation of the liquid crystal molecules increases if the voltage difference becomes larger, thereby making the corresponding color brighter.
Likewise, as shown in FIG. 6, if the common voltage Vcom2 flowing in the common line due to the parasitic coupling decreases the applied common DC voltage Vcom1, the voltage difference V3 between the voltages Vdata and Vcom2 applied to the green pixel region is greater than the voltage difference V4 between the voltages Vdata and Vcom2 applied to the red and blue pixel regions. Accordingly, a greenish tint results because the color green is brighter than the other colors. Furthermore, since the liquid crystal molecules are rotated unstably due to distortion of the common voltage Vcom, residual images corresponding to previous images are generated during conversion of images. Flicker of the images is also generated, which deteriorates picture quality.